Runx2 regulation during development and regeneration of bone in the zebrafish (Danio rerio)

A key feature of vertebrates is the development of bone. In mammals the gene Runx2 is considered a major switch in bone development, as knockout mice for Runx2 never develop any bone. We cloned two zebrafish Runx2 homologues, Runx2a and Runx2b, and assessed their roles in zebrafish. The homologues are 80% identical, but they are both more than 80% identical to Runx2 in other species and Runx2a is 10% more identical to tetrapod Runx2 than Runx2b is. We wanted to find out whether the sequence diversification of the zebrafish homologues led to function diversification. Two promoters control the expression of Runx2. Each promoter results in a different messenger start. We assessed promoter activity by performing real time quantitative PCR. For both genes, one of the promoters is much more active than the other and between genes the promotor activity is different. During development both genes are expressed simultaneously in developing bony structures in the head. In the pectoral fin however, Runx2a is expressed in the fin bud and Runx2b in elements at the base of the fin. The adult pectoral fin and its bony elements can regenerate after injury. During this process, induced by fin-clipping, Runx2b expression was found in the fin itself. From this we conclude that the two Runx2 homologues in zebrafish are both active in bone development and regeneration, but have acquired slightly different functions. In addition we find that development and regeneration are not regulated by identical pathways.